Caption: Stem cell. Coloured scanning electron micrograph (SEM) of a human embryonic stem cell (HESC, orange). HESCs are pluripotent, that is they are able to differentiate into any of the 200 cell types in the human body. The type of cell they mature into depends upon the biochemical signals received by the immature cells. This ability makes HESCs a potential source of cells to repair damaged tissue in diseases such as Parkinson's and insulin-dependent diabetes. However, research using HESCs is controversial as it requires the destruction of an embryo.